The present invention relates to a thermal printer and printing method thereof, and more particularly, to a thermal printer and printing method for compensating for picture quality deterioration due to a common drop and a temperature characteristic of a thermal print head.
In general, examples of an apparatus for printing using a thermal print head (TPH) include a thermal printer, a color copier, a facsimile machine, etc. Among these, a sublimation-type thermal printer prints a desired image or picture according to the amount of dye transferred to a sheet of recording paper, by applying energy to the TPH and sublimating the dye contained on a dye-deposited film by the energy emitted from the TPH.
A conventional thermal printer, as shown in FIG. 1, stores one frame of image data to be printed in its frame memory 10. When printing starts, the frame memory 10 transfers one line of the image data to be printed to a line memory 20 and to a first selection contact point a0 of a controlling switch 51.
The one line of image data to be printed is synchronized with the clock generated in a clock generator 31 and stored in the line memory 20 according to the address generated in an address counter 32. Gradation counter 33 generates gradation data having a value from 0-255, given that the image data is expressed in eight-bit form, and outputs the gradation data as an input signal to a comparator 34.
When data is read from line memory 20 and actually printed by the TPH 40, the data is printed according to its value and the gradation level. For example, if image data consists of eight bits, gradations of the printing of a single pixel can vary in value 0 to 255, and the TPH 40 can be made to print up to 255 times with respect to each pixel.
The gradation counter 33 increments in value from 1 to 255. Then, the output of gradation counter 33 and the eight bit image data of line memory 20 are compared in the comparator 34. As the result thereof, the output of comparator 34 becomes "high" or "low," thereby determining whether the dots of TPH 40 are to emit heat or not. Thus the gradation of the printed pixel will correspond to the value of the eight bit image data on a scale set by the gradation counter.
Controlling switch 51, dot number computing memory 52, dot number computing controller 53, common drop correcting ROM 54, and strobe signal generator 55 constitute a common drop correcting unit 50 for compensating picture quality deterioration due to a common drop of TPH 40. Analog-to-digital converter 61, temperature correcting ROM 62, and power source 63 composed of a switching mode power supply (SMPS) and a detecting temperature thermistor (neither being shown in detail) attached to the back side of the heating element substrate (see FIG. 2) of TPH 40, constitute a temperature correcting unit 60 for compensating for picture quality deterioration due to TPH temperature change.
Common drop of a TPH is understood to mean the generation of a voltage drop due to the parasitic resistance components present within the TPH 40. If the energy applied to the dots of the TPH 40 is varied by the voltage drop, the picture quality will deteriorate.
In other words, assuming that reference letter V represents the voltage applied to the respective heating elements of the TPH, and reference letter T represents the time during which heat is applied, the applied energy E can be expressed by the following equation. ##EQU1##
The common drop phenomenon has a characteristic such that the value of the voltage drop is nearly proportional to the number of the simultaneously heated dots in one line of the TPH 40; that is, the greater the number of simultaneously heated dots in a line of the TPH, the greater the voltage drop within TPH 40. Accordingly, the energy applied to the dots of the TPH 40 becomes smaller in effect, and thereby the printing density is lowered, such that printing is dimmer than for the case where fewer dots are simultaneously heated. Common drop correcting unit 50 corrects picture quality deterioration caused by the problem of common drop, by adjusting the heating period of a strobe signal which is based on the above mentioned proportional relationship between the common drop and the number of the simultaneously heated dots.
The TPH 40 performs printing by converting electrical energy into thermal energy through a resistance. Even if the same amount of electrical energy is applied, since the heat actually generated in the respective dots of TPH 40 varies with ambient temperature fluctuations and with a heat accumulation phenomenon occurring in the thermal print head, the printing density is varied. To correct the picture quality deterioration due to temperature changes in the TPH, a thermistor is installed on the back side of the heat element substrate of TPH 40 to detect the temperature of TPH 40. The detected temperature therein is converted to digital temperature data in analog-to-digital converter 61. Compensation data for detected temperature values of TPH 40 are stored in the temperature correcting ROM 62. Thereafter, compensation data for the detected temperature is obtained from the temperature correcting ROM, and the SMPS of power source 63 changes the voltage applied to TPH 40 according to the stored temperature data and thereby changes the applied energy of TPH 40.
In other words, the SMPS changes the voltage applied to TPH 40 in accordance with the input temperature data. For example, picture quality deterioration due to a temperature change is prevented by lowering the voltage if the temperature is high, or increasing the voltage if the temperature is low.
However, the temperature correcting unit 60 for correcting the TPH temperature requires a controlling circuit which can change the voltage according to the temperature dam input to the SMPS of power source 63 and further requires a connector for transmitting the temperature dam.